1. Field of the Invention
The invention relates to a gas purifying catalyst, and, more particularly, to a burned gas purifying catalyst for use with an exhaust system of an automobile engine suitable for emission level controls of nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO).
2. Description of the Related Art
As one of catalysts installed in an exhaust line of an automobile engine to purify the exhaust gas or to significantly lower emission levels of oxides of nitrogen (NOx) as well as hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas an automobile exhaust control catalyst, there has been a monolith type catalytic convertor which is formed with an under catalyst layer having active alumina particles and platinum (Pt) and rhodium (Rh) on a monolith honeycomb carrier and an over catalyst layer, coated over the under catalyst layer, which comprises barium-fixed ceria (cerium oxide) particles, active alumina particles and palladium (Pd). Such a catalyst is known from, for instance, Japanese Unexamined Patent Publication No. 3-207446. The reason for fixing barium (Ba) to the ceria particles is to prevent the ceria from suffering heat deterioration. The barium-fixed ceria particles are produced in such a manner to dry and solidify a mixture of a barium nitrate solution with ceria particles as a solid lump of barium nitrate-adsorbed ceria particles and break it into particles. The over catalyst layer is coated by dipping a catalyst carrier in a slurry of a palladium chloride solution with the barium nitrate-adsorbed ceria particles and, thereafter, dry and burn the slurry film on the catalyst carrier.
While barium (Ba) is essentially intended to play as an agent to prevent heat deterioration of ceria, it is in some cases used to purify exhaust gases, in particular to reduce nitrogen oxides (NOx) in exhaust gases as is known from, for instance, Japanese Unexamined Patent Publication No. 7-108172. The catalyst described in the Japanese Unexamined Patent Publication No. 7-108172 is a monolith honeycomb type catalytic convertor that carries an under catalyst layer having barium (Ba) supported by an alumina support and an over catalyst layer having platinum (Pt) and rhodium (Rh) supported by an alumina support. This catalyst reduces nitrogen oxides (NOx) through the steps of oxidizing nitrogen oxides (NOx) with the barium (Ba) in the over catalyst layer, a lowering the concentration of oxygen (O.sub.2) in the exhaust gas so as to produce a reducing atmosphere in which the nitrogen oxides (NOx) are separated from the barium, and reducing the nitrogen oxides (NOx) by the catalytic metal in the under catalyst layer making the utilization of hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas as reducing agents.
A typical problem the exhaust gas emission control catalysts experience is sulfur (S) poisoning and/or water (H.sub.2 O) poisoning and is significant in particular if they contain large amounts of barium which has a strong tendency to be poisoned. It has been proved by the inventors of this invention that lanthanum (La) has a tendency of the sulfur (S) poisoning and/or water (H.sub.2 O) poisoning as well. Accordingly, unless the catalyst containing platinum and rhodium or palladium as main catalytic metals is kept away from the sulfur (S) poisoning and water (H.sub.2 O) poisoning, it is difficult that the catalyst maintains its intended emission control efficiency for a long period of time.